In our joint study we have identified insulin and lactogen receptors in rat liver Golgi vesicles. We have hypothesized that they are precursors for the plasma membrane receptors but do not exclude the possibility that they play some kind of intracellular role. We intend to evaluate the precursor hypothesis especially by: (1) Characterizing and comparing binding to plasma membrane and Golgi receptors in respect to kinetics, specificity, affinity, influence of pH and buffer composition, and enzyme digestions. (2) Determining subcellular distribution of receptors for insulin and lactogen in quantitative terms, and localizing 125I-hormone by EM radioautography. (3) Evaluating subcellular distribution of receptors in different physiological states. (4) Determining the time course of change, in subcellular fractions, of lactogen receptors during induction and rapid disappearance of receptor. The ability to induce receptor with estrogen in the male or by a pituitary implant in the hypophysectomized rat, as well as our converse ability to abolish these receptors with hypophysectomy of the female rat will facilitate our capacity to analyze the kinetics of receptor appearance and disappearance in the subcellular fractions. Such data should provide the basis for critically evaluating the precursor hypothesis and could provide clues as to other functions of the Golgi receptors.